A reoccurring challenge associated with the manufacture of optical systems concerns the coupling of the optical signals between the optical fibers used for transmission over distances and the optical devices that generate, detect, analyze and/or otherwise manipulate the optical signals. The source of the challenge is associated with the small beam sizes characteristic of the fiber optic systems. For example, the typical mode size of an optical signal traveling through single mode optical fiber is about ten micrometers (xcexcm) in diameter.
Consider, for example, the problem of coupling an optical signal emitted from an optical signal port such as an endface of an optical fiber. The beam is typically diverging from the fiber. Therefore, some sort of aligned collimating/focusing optics is required to improve the collimation of the emitted beam or bring the beam to a waist in the optical system of the optical device. Contrastingly, when coupling optical signals from an optical system into an optical fiber, beam sizes of, for example, 100""s of micrometers in diameter must be focused down to the approximately 10 xcexcm diameter to be efficiently coupled into the fiber endface input port. Moreover, the beam spot must be aligned with the approximately 10 xcexcm core of the optical fiber.
A further problem associated with manufacturing/packaging surrounds the sealing of optical system of the optical device. To ensure the long-term stability, optical systems are typically hermetically sealed. This requirement is especially true for active optical components, such as lasers due to their high-localized operating temperatures, for example. In the context of micro-optical electrochemical systems (MOEMS), hermetic sealing is required to ensure the continued operation of the devices.
The typical approach to solving the problem of hermetically sealing the optical system while providing for the transmission of the optical signal across the hermetic boundary is to insert an optical fiber through a fiber feedthrough in the hermetic package and then secure the endface of the optical fiber down onto a submount or bench in the packages. This allows for the control of optical beams propagating between the optical system and the optical signal port of the fiber endface because of the fixed relationship between the endface and the optical components of the optical system. Hermeticity is ensured by sealing the package fiber feedthrough around the optical fiber.
This conventional strategy for using the optical fiber to convey the optical signal across the hermetic boundary is common in optical systems providing low levels of integration. Optical systems typically have only one or two fiber pigtails. As a result, only a few fiber feedthroughs must be sealed.
This strategy for conveying the optical signals across the hermetic boundary becomes unworkable, however, in modem wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) systems. Such systems as WDM/DWDM switches, multplexers/demultiplexers, and add/drop devices typically require that many optical signals be brought into a single hermetically sealed package. Robust sealing is thus required for every fiber feedthrough port in the hermetic package.
Another problem associated with the use of fiber to transmit the optical signals across the hermetic boundaries surrounds the particular manufacturing requirements for commercial-grade fiber optic systems. Generally, there can be no residual organic compounds remaining in the hermetic package after its sealing. Thus, epoxies cannot be used to bond the optical elements, such as the fiber endfaces within the hermetic package. Moreover, any fluxes used during the manufacturing of the optical device must be rigorously cleaned from the package prior to lid sealing. Thus, the fiber endface fixturing techniques must be organics-free.
The present invention concerns a technique for fixturing optical fiber endfaces and transmitting optical signals across a hermetic boundary from an optical system, for which the optical fibers function as optical signal ports. The invention, however, also has relevance to applications where the function of the external optical signal port is provided by another optical system, without intervening fiber optics. Specifically, the optical signals are transmitted into and/or out of the optical system across the hermetic boundary as beams that are transmitted through a window structure in the walls of the hermetic package.
In general, according to one aspect, the invention features a system for coupling optical signal beams through a hermetic package of an optical device. The system comprises a device bench and a hermetic package surrounding an optical system that is installed on the bench. A fixture is used for securing optical signal ports to the bench outside of the hermetic package. A window structure is provided in the wall of the hermetic package that enables optical signal beams to be transmitted between the optical system that is inside of the hermetic package and external optical signal ports.
In the present embodiments, the optical signal ports are the optical fiber endfaces. These optical fiber endfaces can either be the source of the optical signal beams, such as where optical signal beams are entering the package to be switched to another optical fiber. IN other cases, the optical signal ports can receive the optical signals as in the case where a beam is emitted from the optical system after being switched, for example. In still other instances, the optical signal ports are bi-directional, both emitting and receiving optical signal beams.
Generally, however, the ports, rather than being fiber optic based, could be ports to another optical system. For example, two optical systems could be manufactured on a common bench, but separately sealed in their respective hermetic packages. Optical signal beams are transmitted through the inventive window structures between the separate systems.
According to the preferred embodiment, a lens array is used for coupling the optical signal beams between the optical signal ports and the optical system. Such lens arrays can be located outside the package as shown, or on the hermetically sealed side of the window structure. The lens arrays can be conveniently formed as a series of discrete lenses on a common substrate. This is preferably generated in a monolithic fashion. Further, these are preferably micro-optic structures having lens diameters of less than 1,000 micrometers. In the preferred embodiment, the lens diameters are about 200 to 500 micrometers or smaller.
In the preferred embodiment, the optical signal ports are endfaces of optical fibers. They are preferably secured to the bench in a fiber block. This block currently has a two-part construction with V-grooves for securing the optical fiber endfaces in a stable relationship relative to the bench and thus, the optical system.
On problem, especially with bi-directional systems surrounds back reflections from the fiber enfaces. In one embodiment, this problem is solved by forming wedge-shaped endfaces on the optical fibers. The resulting beam tilt can be solved with a beam tilt compensator that directs the beams to propagate parallel to the optical bench.
In general, according to another aspect, the invention also features a collimator for an optical device. This collimator comprises a substrate. A two-dimensional array of lenses is formed on the substrate. This allows the formation of two-dimensional arrays of beams, which are useful in modern tip/tilt mirror switching optical systems, for example.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.